Electrode fixation in interventional medical systems

ABSTRACT

An implantable medical device assembly includes a mounting structure, an electrode protruding from a surface of the structure, between opposing sides thereof, and tissue-penetrating fixation tines, each extending from a corresponding shoulder of the structure surface, adjacent to the opposing sides. In a relaxed condition, each tine extends away from the surface and then bends toward a proximal end of the structure and back toward the surface. In an extended condition, each tine bends toward a distal end of the structure and extends along the corresponding shoulder. A holding member of a delivery tool has opposing sidewalls defining a cavity, wherein each sidewall includes a rail-like edge that fits in sliding engagement with a corresponding shoulder, to deform a corresponding tine into the extended condition, when an operator passes the assembly into the cavity. Applying a push force, to move the assembly back out form the cavity, releases the tines.

TECHNICAL FIELD

The present invention pertains to interventional medical systems, and,more specifically, to implantable electrode fixation at a stimulationsite.

BACKGROUND

An implantable medical device, for the delivery of stimulation therapy,may include an electrode and a fixation component configured to hold theelectrode in intimate contact with tissue at a stimulation site. Onetype of such a device may be a traditional implantable cardiac pacemakerthat includes a pulse generator and a pacing electrode coupled to thegenerator by an elongate insulated lead wire. The pulse generator istypically implanted in a subcutaneous pocket, remote from the heart,with the lead wire extending therefrom to a pacing site where theelectrode is positioned. Another type of implantable medical device maybe one wholly contained within a relatively compact package for implantin close proximity to the pacing site. FIG. 1 illustrates such a device100 including an hermetically sealed housing 105, preferably formed froma biocompatible and biostable metal such as titanium, that contains anelectronic controller and associated power source (not shown), to whichat least one electrode 111 is coupled, for example, by a hermeticfeedthrough assembly (not shown).

With further reference to FIG. 1, device 100 has been deployed by anoperator via a delivery tool 200, which the operator has maneuvered upthrough the inferior vena cava IVC and across the right atrium RA intothe right ventricle RV. The deployed device 100 is shown fixed at thepacing site by a fixation member 115 thereof, for example, includingtissue-penetrating tines that surround electrode 111 and secureelectrode 111 in intimate contact with tissue at the site. Furtherdescription of a suitable construction for device fixation member 115may be found in the co-pending and commonly assigned United StatesPatent Application having the pre-grant publication number 2012/0172690A1.

An alternative pacing site may be located on an epicardial surface ofthe heart, for example, on the left side of the heart for theapplication of pacing therapy to treat heart failure. FIG. 2 is aschematic showing an access site A for creating a passageway between apatient's diaphragm 19 and xiphoid process 20 of sternum 13, forexample, to implant a pacing electrode on an epicardial surface 6 of thepatient's heart, which is enclosed within the pericardial sac 15. Aftermaking a superficial incision, an operator may open a passageway betweendiaphragmatic attachments 18 and diaphragm 19 by using blunt dissectiontools and techniques that are known in the art. Then, the operator mayemploy a piercing tool to pass a guide wire through the pericardial sac15, also according to methods known in the art. The operator may usefluoroscopic guidance to position a distal portion of the guide wirealong a portion of epicardial surface 6, at which a target site islocated, and then pass a guiding sheath over the positioned guide wire.The guiding sheath then serves as a conduit for delivery of theimplantable electrode to the target site. In this context, to deliverand then fix, or secure the implantable electrode at an epicardial site,there is a need for new configurations of interventional systems andassociated implantable device assemblies.

SUMMARY

An implantable medical device assembly, according to some embodiments ofthe present invention, includes: a mounting structure having opposingsides that define a width thereof, proximal and distal ends that definea length thereof, and a surface that extends between the opposing sidesand proximal and distal ends, and that has shoulders formed therein,each being adjacent a corresponding side of the structure; an electrodebeing approximately centered between the opposing sides and protrudingfrom the surface of the mounting structure; and first and secondtissue-penetrating fixation tines, each of which extends from acorresponding shoulder of the mounting structure surface. Each tine iselastically deformable from a relaxed condition to an extendedcondition, wherein, in the relaxed condition, each tine extends awayfrom the mounting structure surface and then bends toward the proximalend of the structure and back toward the surface so that a piercing tipof each tine is located proximal to the electrode, and wherein, in theextended condition, each tine bends toward the distal end of thestructure and extends along the corresponding shoulder of the structure.

In some embodiments, each fixation tine is a component formed from asuper-elastic wire, and the component may include a pre-formed L-shapedsegment and a pre-formed V-shaped segment terminated by the piercingtip. The segments may bend in opposite directions but in a single plane,the same for both. In the above described assembly, the mountingstructure may include first and second internal channels, wherein theL-shaped segment of each fixation tine is mounted in a correspondinginternal channel so that the corresponding V-shaped segment extends awayfrom the corresponding shoulder.

An interventional medical system of the present invention, according tosome embodiments, includes the above described device assembly and adelivery tool, wherein the delivery tool includes a holding member withopposing sidewalls defining a cavity sized to hold the device assemblymounting structure therein, and wherein each sidewall includes arail-like edge that fits in sliding engagement with a correspondingshoulder of the mounting structure and deforms a corresponding fixationtine of the device assembly into the extended condition. According tosome methods of the present invention, an operator passes the proximalend of the mounting structure through a distal opening of the holdingmember cavity, until the rail-like edges come into sliding engagementwith respective shoulders of the mounting structure; and then theoperator continues to pass the mounting structure into the cavity, toload the device assembly therein, so that each engaged rail-like edgeelastically deforms the corresponding tissue-penetrating fixation tinefrom a relaxed condition to an extended condition. After the operatorpositions the holding member and loaded device assembly at a stimulationsite, for example, on an epicardial surface of a patient's heart, theoperator can release the fixation tines of the device assembly from theextended condition, to engage with tissue at the site, by applying apush force against the mounting structure of the device assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments will hereinafter be described in conjunctionwith the appended drawings wherein like numerals/letters denote likeelements, and:

FIG. 1 is a schematic showing an exemplary implant of a relativelycompact medical device;

FIG. 2 is a schematic depicting a sub-sternal access site through whichan implantable electrode may be passed for securing to an epicardialsurface of a patient's heart;

FIGS. 3A-B are a plan view and an end view of an implantable medicaldevice assembly, according to some embodiments;

FIG. 3C is a cross-section view through section line C-C of FIG. 3B,according to some embodiments;

FIGS. 4A-B are a plan view and an end view of a tissue-penetrating tinecomponent, according to some embodiments;

FIGS. 5A-B are a plan view and an end view of a delivery tool which maybe included together with embodiments of device assemblies in aninterventional medical system, according to some embodiments;

FIG. 5C is a perspective view of a portion of the delivery tool and thedevice assembly together, according to some embodiments and methods;

FIG. 6 is a schematic showing the delivery tool positioned for securingan electrode of the medical device assembly at an epicardial site,according to some methods;

FIG. 7 is a schematic depicting release of tissue-penetrating fixationtines in the interventional medical system, according to someembodiments; and

FIG. 8 is a plan view of an exemplary alternate embodiment of the deviceassembly.

DETAILED DESCRIPTION

The following detailed description is exemplary in nature and is notintended to limit the scope, applicability, or configuration of theinvention in any way. Rather, the following description providespractical examples, and those skilled in the art will recognize thatsome of the examples may have suitable alternatives.

FIGS. 3A-B are a plan view and an end view of an implantable medicaldevice assembly 300, according to some embodiments. FIGS. 3A-Billustrate device assembly 300 including a mounting structure 310 havinga length defined from a proximal end 31P to a distal end 31D thereof, athickness defined from a first surface 311 to a second surface 312thereof, and a width defined from a first side 301 to a second side 302thereof. FIGS. 3A-B further illustrate assembly 300 including anelectrode 32 protruding from first surface 311 of structure 310, andfirst and second tissue-penetrating fixation tines 341, 342 located oneither side of electrode 32, which may be approximately centeredtherebetween. According to preferred embodiments, tines 341, 342 areformed from a super-elastic material and are configured to secureelectrode 32, in intimate tissue contact, at a stimulation site on anepicardial surface of a patient's heart. To conform to the epicardialsurface, first surface 311, being that which confronts the epicardialsurface, preferably has a concavity, for example, with a radius ofcurvature (indicated with dashed lines in FIG. 3A) of between about 1inch and 6 inches, formed therein. According to one exemplaryembodiment, the concavity of surface 311 is centered on mountingstructure 310 and has a radius of approximately 3.5 inches set intosurface 311 by about 0.015 inch, wherein electrode 32 is located on acenterline of the concavity.

In some embodiments, when device assembly 300 forms a relatively compactimplantable medical device, for example, similar to device 100 describedabove in conjunction with FIG. 1, mounting structure 310 defines ahermetically sealed enclosure (indicated with dashed lines in FIG. 3B),which is sized to hold an electronic controller and associated powersource for coupling to electrode 32, for example, via a hermeticallysealed feedthrough assembly like that known to those skilled in the art.In these embodiments, as well as in others, structure 310 may be formedfrom a biocompatible and biostable metal, such as titanium, incombination with an overlay of a biocompatible and biostable polymer,such as parylene, polyimide, or urethane, for electrical isolation.Electrode 32 may be constructed from any suitable material and by anysuitable method known to those skilled in the art of medical electricalcardiac pacing.

With further reference to FIG. 3B, mounting structure 310 includesshoulders 311-S formed in first surface 311, wherein each shoulder 311-Sis aligned along the length of structure 310 and located adjacent acorresponding side 301, 302 of structure 310, and wherein each tine 341,342 extends away from a corresponding shoulder 311-S. Each tine 341, 342includes a piercing tip 40, and FIG. 3A shows tines 341, 342 in arelaxed condition, bending toward proximal end 31P and first surface 311of structure 310, so that each piercing tip 40 is located proximal toelectrode 32. Each tine 341, 342 may be secured to structure 310 byextending within a corresponding internal channel 318 thereof, each ofwhich is located distal to electrode 32, for example, as shown in FIG.3C, which is a cross-section view through section line C-C of FIG. 3B,according to some embodiments. FIG. 3C illustrates tine 341 including apre-formed L-shaped segment 4L, which extends within, and interlockswith internal channel 318, and mounting structure 310 including a crosshole 3 (which may extend from first side 301) in communication withchannel 318 to receive injection of an adhesive filler that furthersecures segment 4L in channel 318. Alternately, or in addition,auxiliary mechanical interlocking between pre-formed L-shaped segment 4Land channel 318 may be included. FIG. 3C further illustrates tine 341including a pre-formed V-shaped segment 4V, which extends from L-shapedsegment 4L outside channel 318 and away from shoulder 311-S to definethe above-described relaxed condition of tine 341. It should beunderstood that tine 342, according to the illustrated embodiment, alsoincludes pre-formed L-shaped and V-shaped segments 4L, 4V that extendinside and outside, respectively, a corresponding channel 318 locatedadjacent to second side 302 of structure. However, according toalternate embodiments, in which the means for mounting tines 341, 342 isvaried, what is herein designated as the L-shaped segment 4L of theillustrated embodiment can be pre-formed into any other suitable shapethat conforms to alternate mounting means.

FIG. 4A is a plan view of either of tissue-penetrating fixation tines341 342 as a separate component from assembly 300, according to someembodiments. FIG. 4A illustrates tine 341, 342 including theaforementioned pre-formed L-shaped and V-shaped segments 4L, 4V, whereinL-shaped segment 4L extends from a first end 4L1 thereof to a second end4L2 thereof around a bend that encloses a 90 degree angle, and V-shapedsegment 4V extends from second end 4L2 of L-shaped segment 4L topiercing tip 40. FIG. 4A further illustrates V-shaped segment 4V bendingin an opposite direction from L-shaped segment 4L, and including afirst, relatively straight, portion 4V-1, a second, arched, portion4V-2, and a third, relatively straight, portion 4V-3, wherein secondportion 4V-2 connects first and third portions 4V-1, 4V-3, and thirdportion 4V-3 is terminated by piercing tip 40. A length of third portion4V-3, for example, about 0.12 inch, sets a depth to which each tine 341,342 can ‘bite’ into tissue at an implant site; and a length of secondportion 4V-2, for example, about 0.08 inch around a radius of about 0.03inch, adds a bit of depth to the bite and determines how much tissue isencompassed in the ‘bite’ of each tine 341, 342. The release of tines341, 342 for ‘biting’ is described in greater detail below, inconjunction with FIG. 7.

FIG. 4B is an end view of tine component 341, and with reference to FIG.4B in conjunction with FIG. 4A, each tine component 341, 342 has agenerally rectangular axial cross-section that is uniform along bothsegments 4L, 4V, sans piercing tip 40, wherein a single plane in whichboth segments 4L, 4V bend is orthogonal to longer sides of the axialcross-section. According to an exemplary embodiment, tine components341, 342 may be formed from a rolled Nitinol wire (e.g., having adiameter of approximately 0.012 inch, prior to rolling), and piercingtip 40 is formed by a first angled surface cut in one of the longersides of the axial cross-section, according to an angle 7 (FIG. 4A), forexample, of about 25 degrees. Each of tine components may also include asecond angled surface cut into one of the shorter sides of the axialcross-section, according to an angle β (FIG. 4B), for example, of about60 degrees, wherein, with further reference to FIG. 4B, in conjunctionwith the end view of FIG. 3B, the second angled surfaces of tines 341,342 face generally toward one another in assembly 300, such that thedashed line in FIG. 4B represents the second angled surface of tine 342.

V-shaped segment 4V of each fixation tine 341, 342 is elasticallydeformable from the illustrated relaxed condition to an extendedcondition, in which each segment 4V bends toward distal end 31D ofstructure 310 and extends along the corresponding shoulder 311-S, forexample, as described below in conjunction with FIGS. 5C and 6. Tines341, 342 may be held in the extended condition until an operatorpositions assembly 300 in proximity to a stimulation site, after whichthe operator may release tines 341, 342 from the extended condition sothat piercing tips 40 ‘bite’ into tissue adjacent the site, therebysecuring electrode 32 in intimate tissue contact for stimulationtherapy. FIGS. 5A-B are a plan view and an end view of a delivery tool500, which may be employed by the operator to hold device assembly 300,with tines 341, 342 in the extended position, and to position assembly300, and then to release tines 341, 342 to secure electrode 32,according to some embodiments of interventional medical systems.

FIGS. 5A-B illustrate delivery tool 500 including an elongate shaft 510and a holding member 530 attached to a distal end 512 of shaft 510,wherein first and second opposing sidewalls 531, 532 of holding member530 define a cavity 535 therebetween, which is sized to hold deviceassembly 300 therein. The end view of FIG. 5B is looking into a distalopening 53 of cavity 535, which may be sized to receive passage ofdevice assembly 300 therethrough, for example, as shown in theperspective view of FIG. 5C. FIG. 5B illustrates each holding membersidewall 531, 532 including a rail-like edge 531-E, 532-E configured tofit in sliding engagement with a corresponding shoulder 311-S of deviceassembly mounting structure 310 (FIG. 3B), for example, as an operatorpasses proximal end 31P of device assembly mounting structure 310 intoholding member cavity 535, through distal opening 53 thereof. Thus, withreference to FIG. 5C, as the operator continues to pass mountingstructure 310 into cavity 535, per arrow L, to load device assembly 300therein, each engaged rail-like edge 531-E, 532-E elastically deformsthe corresponding tissue-penetrating fixation tine 341, 342 from therelaxed condition to the extended condition, according to arrow X andthe dashed lines of FIG. 5C. Thus, when assembly 300 is loaded inholding member 530, each tine 341, 342 bends toward distal end 31D ofstructure 310 and extends along the corresponding shoulder 311-S. Withdevice assembly 300 loaded into holding member 530, the operator canposition holding member 530 at an epicardial site, for example, bypassing delivery tool 500 into the pericardial space via sub-xiphoidaccess (e.g., access site A of FIG. 2).

According to an exemplary embodiment, shaft 510 of delivery tool 500,for example, extending over a length of approximately 30 cm to 35 cm,may be formed by a stainless steel braid-reinforced medical gradepolymer of one or more appropriate grades of polyether block amide(e.g., PEBAX® 6333 and 7033); and holding member 530 of tool 500 may beformed from an appropriate grade of polyether block amide (e.g., PEBAX®7233) and include a radiopaque marker bonded thereto, for example, aPlatinum/Iridium or gold marker, or a polyamide material with aradiopaque filler, such as Tungsten-filled Vestamid®.

FIG. 6 is a schematic showing delivery tool 500 positioned for securingelectrode 32 of device assembly 300 at the epicardial site, according tosome methods, for example, to provide pacing stimulation. FIG. 6illustrates a guiding sheath 700 providing a passageway for theinsertion of delivery tool 500 into the pericardial space, betweenepicardial surface 6 and pericardial sac 15, through access site A,which may be formed by any suitable method known in the art, forexample, as described above in conjunction with FIG. 2. Fluoroscopic orvideo monitoring may be employed for guidance in positioning holdingmember 530.

With reference back to FIG. 5B, shaft 510 may include a lumen 501extending longitudinally from a proximal end 511 to distal end 512 ofshaft 510, and being in fluid communication with holding member cavity535. FIGS. 5A-B and FIG. 6 further illustrate an ejector rod 540extending in sliding engagement within shaft 510 of delivery tool 500,for example, inserted through lumen 501 by the operator, afterpositioning holding member 530 and loaded device assembly 300 at theepicardial site and removing the guide wire. According to theillustrated embodiment and some methods, the operator may apply a pushforce to mounting structure 310 of the loaded and positioned deviceassembly 300 through rod 540, per arrow P, to move device assembly 300out through distal opening 53 of cavity 535, thereby releasing fixationtines 341, 342 to ‘bite’ into tissue at the epicardial site. FIG. 7 is aschematic depicting the release of fixation tines 341, according to someembodiments. FIG. 7 illustrates incremental positions A-H of assembly300 relative to holding member 530 of delivery tool 500, whereinposition A corresponds to device assembly 300 loaded within holdingmember 530 so that tines 341, 342 are in the extended condition, andpositions B-H correspond to the release of tines 341, 342 as piercingtips 40 thereof ‘bite’ into tissue for securing electrode 32 to astimulation site. An approximate ‘bite’ depth BD of tines 341, 342 maybe about 0.132 inch (3.3 mm), as dictated by the above disclosedexemplary radius and lengths of tine portions 4V-2 and 4V-3 (FIG. 4A).Once tines 341, 342 are fully engaged with the tissue, for example,being generally at position H, the operator can retract delivery tool500 from the pericardial space to leave device assembly 300 implanted atthe epicardial site.

With reference back to FIGS. 5B-C, holding member edges 531-E, 532-E intool 500 preferably define a longitudinally extending slot 53-Stherebetween so that, when device assembly 300 is held in cavity 535,electrode 32 extends through slot 53-S. Thus, electrode 32 can makecontact with tissue at the stimulation site prior to the release oftines 341, 342, so that an operator can test electrode function at thesite prior to securing electrode 32 to the site, according to somemethods. With further reference to FIG. 5C, first surface 311 of deviceassembly mounting structure 510 is shown including a conductive area 35surrounding electrode 32, and being isolated therefrom, to form anotherelectrode for bipolar function with electrode 32. Alternately,conductive area 35 is located opposite electrode 32, on second surface312 of mounting structure 310. According to some exemplary embodimentsof device assembly 300, in which mounting structure 310 is formed by ametal and polymer overlay, as described above, conductive area 35 may beformed by removing a portion of the polymer overlay from first surface311 or second surface 312. According to some alternate embodiments, adevice assembly 800, for example, as shown in the plan view of FIG. 8,includes an electrode 82 as part of an elongate electrode subassembly820 that extends from distal end 31D of mounting structure 310, to formthe bipolar pair with electrode 32, in lieu of conductive area 35. In anexemplary embodiment, electrode subassembly 820 includes an insulatedconductor 821 electrically coupled, via a hermetically sealedfeedthrough (not shown), to the aforementioned controller and powersource enclosed within mounting structure 310, wherein electrode 82 ismounted about insulated conductor 821 and electrically coupled thereto.Electrode subassembly 820 may be constructed from any suitable materialsand according to any suitable methods known to those skilled in the artof implantable medical electrical leads.

In the foregoing detailed description, the invention has been describedwith reference to specific embodiments. However, it may be appreciatedthat various modifications and changes can be made without departingfrom the scope of the invention as set forth in the appended claims.

1.-20. (canceled)
 21. An implantable medical device assembly comprising:a mounting structure having a length defined from a proximal end thereofto a distal end thereof, the mounting structure comprising at least oneshoulder formed in a surface of the structure and aligned along aportion of the length of the mounting structure; and atissue-penetrating fixation tine corresponding to the shoulder, thetissue-penetrating fixation tine being elastically deformable from afirst position configured to fix the implantable medical device totissue at a stimulation site to a second position configured to allowdelivery of the implantable medical device to the stimulation site, inthe first position a portion of the tissue-penetrating fixation tinebends toward the proximal end of the mounting structure and out of theshoulder of the mounting structure, and in the second position theportion the tissue-penetrating fixation tine bends toward the distal endof the mounting structure and extends along and within the shoulder ofthe mounting structure.
 22. The assembly of claim 21, wherein: themounting structure further comprises an internal channel; thetissue-penetrating fixation tine comprises a segment extending in theinternal channel of the mounting structure; and the mounting structurefurther comprises adhesive to secure the tissue-penetrating fixationtine in the internal channel.
 23. The assembly of claim 22, wherein theportion of the tissue-penetrating fixation tine comprises a pre-formedV-shaped portion extending in the internal channel of the mountingstructure.
 24. The assembly of claim 22, wherein the segment of thefixation tine comprises an L-shaped segment.
 25. The assembly of claim21, wherein the portion of the tissue-penetrating fixation tinecomprises a piercing tip at an end of the tissue-penetrating fixationtine.
 26. The assembly of claim 25, wherein the piercing tip comprisesan angled surface.
 27. The assembly of claim 21, further comprising anelectrode that protrudes from the surface of the mounting structure. 28.The assembly of claim 27, wherein the surface of the mounting structurefurther comprises a conductive area forming another electrode forbipolar function with the electrode that protrudes from the surface ofthe mounting structure.
 29. The assembly of claim 27, wherein themounting structure defines a hermetically sealed enclosure sized to holdan electronic controller and associated power source for coupling to theelectrode.
 30. The assembly of claim 27, wherein the tissue-piercingfixation tine defines a curved shape over or proximal to the electrodein the first position.
 31. The assembly of claim 27, wherein the surfaceof the mounting structure defining a concavity formed therein, theconcavity being centered on the mounting structure, and the electrodebeing located in the concavity.
 32. An interventional medical systemcomprising: an implantable medical device assembly, the assemblycomprising: a mounting structure having a length defined from a proximalend thereof to a distal end thereof, the mounting structure comprisingat least one shoulder formed in a surface of the structure and alignedalong a portion of the length of the mounting structure; and atissue-penetrating fixation tine corresponding to the shoulder, thetissue-penetrating fixation tine being elastically deformable from afirst position to a second position, in the first position a portion ofthe tissue-penetrating fixation tine bends toward the proximal end ofthe mounting structure and out of the shoulder of the mountingstructure, and in the second position the portion the tissue-penetratingfixation tine bends toward the distal end of the mounting structure andextends along and within the shoulder of the mounting structure; and adelivery tool comprising: an elongate shaft extending from a proximalend thereof to a distal end thereof; a holding member attached to thedistal end of the shaft, the holding member including first and secondopposing sidewalls defining a cavity therebetween, the cavity beingsized to hold the device assembly therein, the cavity including a distalopening sized to allow passage of the device assembly therethrough; andwherein, when the device assembly is passed into the cavity of the toolholding member through the distal opening thereof, the portion of thetissue-penetrating fixation tine is deformed into the second position.33. The system of claim 32, wherein the delivery tool further comprisesan ejector rod slideably engaged within the tool to apply a push forceto the device assembly mounting structure when the device assembly isheld in the holding member cavity.
 34. The system of claim 32, whereinthe holding member further comprises a sidewall rail-like edgeconfigured to fit in sliding engagement with the shoulder of the deviceassembly mounting structure.
 35. The system of claim 32, wherein: thedevice assembly further comprises an electrode that protrudes from thesurface of the mounting structure; and a longitudinally extending slotis defined in the tool holding member and when the device assembly isheld in the cavity of the holding member, the electrode of the deviceassembly extends through the slot.
 36. The system of claim 35, whereinthe surface of the device assembly mounting structure includes aconductive area that forms a second electrode for bipolar function withthe device assembly electrode and when the device assembly mountingstructure is held in the cavity of the tool holding member, the secondelectrode is exposed through the slot of the holding member.
 37. Thesystem of claim 35, wherein the mounting structure defines ahermetically sealed enclosure sized to hold an electronic controller andassociated power source for coupling to the electrode.
 38. The system ofclaim 35, wherein the surface of the mounting structure defining aconcavity formed therein, the concavity being centered on the mountingstructure, and the electrode being located in the concavity.
 39. Thesystem of claim 32, wherein the portion of the fixation tine comprises apre-formed V-shaped portion extending in the channel of the mountingstructure.
 40. An implantable medical device assembly comprising: amounting structure extending from a proximal end to a distal end; and atissue-penetrating fixation tine extending from a mounted end coupled tothe mounting structure to a piercing tip, wherein the tissue-penetratingfixation tine is elastically deformable between a storage position and afixation position, wherein, when in the storage position, a segment ofthe tissue-penetrating fixation tine terminating at the piercing tipextends along and within the mounting structure and the piercing tippoints in a first direction, wherein, when in the fixation position, thesegment of the tissue-penetrating fixation tine terminating at thepiercing tip extends out of the mounting structure and the piercing tippoints in a second direction at least partially opposite the firstdirection.